Polished travertine won't absorb a sealant, but honed travertine usually will. Test whether your travertine will absorb a sealant by dropping a small puddle of water on the stone and timing how long it takes to absorb. If the stone darkens within 15 minutes, sealing may prove beneficial.
Showers and other wet areas should not be sealed, since the sealant will prevent evaporation of any water the stone absorbs. Instead, allow your travertine to breathe in this settings.
Sealing travertine and other types of natural stone will not prevent damage from acidic agents, such as cleaning products, orange juice or vinegar. Use pH-neutral cleaning products on travertine.